lvgl/docs/porting/display.md

.. include:: /header.rst 
:github_url: |github_link_base|/porting/display.md

Display interface

To set up a display an lv_disp_buf_t and an lv_disp_drv_t variables have to be initialized.

• lv_disp_buf_t contains internal graphic buffer(s).
• lv_disp_drv_t contains callback functions to interact with the display and manipulate drawing related things.

Display buffer

lv_disp_buf_t can be initialized like this:

    /*A static or global variable to store the buffers*/
    static lv_disp_buf_t disp_buf;

    /*Static or global buffer(s). The second buffer is optional*/
    static lv_color_t buf_1[MY_DISP_HOR_RES * 10];
    static lv_color_t buf_2[MY_DISP_HOR_RES * 10];

    /*Initialize `disp_buf` with the buffer(s) */
    lv_disp_buf_init(&disp_buf, buf_1, buf_2, MY_DISP_HOR_RES*10);

There are 3 possible configurations regarding the buffer size:

1. One buffer LVGL draws the content of the screen into a buffer and sends it to the display. The buffer can be smaller than the screen. In this case, the larger areas will be redrawn in multiple parts. If only small areas changes (e.g. button press) then only those areas will be refreshed.
2. Two non-screen-sized buffers having two buffers LVGL can draw into one buffer while the content of the other buffer is sent to display in the background. DMA or other hardware should be used to transfer the data to the display to let the CPU draw meanwhile. This way the rendering and refreshing of the display become parallel. Similarly to the One buffer, LVGL will draw the display's content in chunks if the buffer is smaller than the area to refresh.
3. Two screen-sized buffers. In contrast to Two non-screen-sized buffers LVGL will always provide the whole screen's content not only chunks. This way the driver can simply change the address of the frame buffer to the buffer received from LVGL. Therefore this method works the best when the MCU has an LCD/TFT interface and the frame buffer is just a location in the RAM.

You can measure the performance of your display configuration using the benchmark example.

Display driver

Once the buffer initialization is ready the display drivers need to be initialized. In the most simple case only the following two fields of lv_disp_drv_t needs to be set:

• buffer pointer to an initialized lv_disp_buf_t variable.
• flush_cb a callback function to copy a buffer's content to a specific area of the display. lv_disp_flush_ready() needs to be called when flushing is ready. LVGL might render the screen in multiple chunks and therefore call flush_cb multiple times. To see which is the last chunk of rendering use lv_disp_flush_is_last().

There are some optional data fields:

• hor_res horizontal resolution of the display. (LV_HOR_RES_MAX by default from lv_conf.h).
• ver_res vertical resolution of the display. (LV_VER_RES_MAX by default from lv_conf.h).
• color_chroma_key a color which will be drawn as transparent on chrome keyed images. LV_COLOR_TRANSP by default from lv_conf.h).
• user_data custom user data for the driver. Its type can be modified in lv_conf.h.
• anti-aliasing use anti-aliasing (edge smoothing). LV_ANTIALIAS by default from lv_conf.h.
• rotated and sw_rotate See the rotation section below.
• screen_transp if 1 the screen can have transparent or opaque style. LV_COLOR_SCREEN_TRANSP needs to enabled in lv_conf.h.

To use a GPU the following callbacks can be used:

• gpu_fill_cb fill an area in memory with colors.
• gpu_blend_cb blend two memory buffers using opacity.
• gpu_wait_cb if any GPU function return, while the GPU is still working LVGL, will use this function when required the be sure GPU rendering is ready.

Note that, these functions need to draw to the memory (RAM) and not your display directly.

Some other optional callbacks to make easier and more optimal to work with monochrome, grayscale or other non-standard RGB displays:

• rounder_cb round the coordinates of areas to redraw. E.g. a 2x2 px can be converted to 2x8. It can be used if the display controller can refresh only areas with specific height or width (usually 8 px height with monochrome displays).
• set_px_cb a custom function to write the display buffer. It can be used to store the pixels more compactly if the display has a special color format. (e.g. 1-bit monochrome, 2-bit grayscale etc.) This way the buffers used in lv_disp_buf_t can be smaller to hold only the required number of bits for the given area size. set_px_cb is not working with Two screen-sized buffers display buffer configuration.
• monitor_cb a callback function tells how many pixels were refreshed in how much time.
• clean_dcache_cb a callback for cleaning any caches related to the display

To set the fields of lv_disp_drv_t variable it needs to be initialized with lv_disp_drv_init(&disp_drv). And finally to register a display for LVGL lv_disp_drv_register(&disp_drv) needs to be called.

All together it looks like this:

    lv_disp_drv_t disp_drv;                 /*A variable to hold the drivers. Can be local variable*/
    lv_disp_drv_init(&disp_drv);            /*Basic initialization*/
    disp_drv.buffer = &disp_buf;            /*Set an initialized buffer*/
    disp_drv.flush_cb = my_flush_cb;        /*Set a flush callback to draw to the display*/
    lv_disp_t * disp;
    disp = lv_disp_drv_register(&disp_drv); /*Register the driver and save the created display objects*/

Here some simple examples of the callbacks:

void my_flush_cb(lv_disp_drv_t * disp_drv, const lv_area_t * area, lv_color_t * color_p)
{
    /*The most simple case (but also the slowest) to put all pixels to the screen one-by-one*/
    int32_t x, y;
    for(y = area->y1; y <= area->y2; y++) {
        for(x = area->x1; x <= area->x2; x++) {
            put_px(x, y, *color_p)
            color_p++;
        }
    }

    /* IMPORTANT!!!
     * Inform the graphics library that you are ready with the flushing*/
    lv_disp_flush_ready(disp_drv);
}

void my_gpu_fill_cb(lv_disp_drv_t * disp_drv, lv_color_t * dest_buf, const lv_area_t * dest_area, const lv_area_t * fill_area, lv_color_t color);
{
    /*It's an example code which should be done by your GPU*/
    uint32_t x, y;
    dest_buf += dest_width * fill_area->y1; /*Go to the first line*/

    for(y = fill_area->y1; y < fill_area->y2; y++) {
        for(x = fill_area->x1; x < fill_area->x2; x++) {
            dest_buf[x] = color;
        }
        dest_buf+=dest_width;    /*Go to the next line*/
    }
}

void my_gpu_blend_cb(lv_disp_drv_t * disp_drv, lv_color_t * dest, const lv_color_t * src, uint32_t length, lv_opa_t opa)
{
    /*It's an example code which should be done by your GPU*/
    uint32_t i;
    for(i = 0; i < length; i++) {
        dest[i] = lv_color_mix(dest[i], src[i], opa);
    }
}

void my_rounder_cb(lv_disp_drv_t * disp_drv, lv_area_t * area)
{
  /* Update the areas as needed. Can be only larger.
   * For example to always have lines 8 px height:*/
   area->y1 = area->y1 & 0x07;
   area->y2 = (area->y2 & 0x07) + 8;
}

void my_set_px_cb(lv_disp_drv_t * disp_drv, uint8_t * buf, lv_coord_t buf_w, lv_coord_t x, lv_coord_t y, lv_color_t color, lv_opa_t opa)
{
    /* Write to the buffer as required for the display.
     * Write only 1-bit for monochrome displays mapped vertically:*/
 buf += buf_w * (y >> 3) + x;
 if(lv_color_brightness(color) > 128) (*buf) |= (1 << (y % 8));
 else (*buf) &= ~(1 << (y % 8));
}

void my_monitor_cb(lv_disp_drv_t * disp_drv, uint32_t time, uint32_t px)
{
  printf("%d px refreshed in %d ms\n", time, ms);
}

void my_clean_dcache_cb(lv_disp_drv_t * disp_drv, uint32)
{
  /* Example for Cortex-M (CMSIS) */
  SCB_CleanInvalidateDCache();
}

Rotation

LVGL supports rotation of the display in 90 degree increments. You can select whether you'd like software rotation or hardware rotation.

If you select software rotation (sw_rotate flag set to 1), LVGL will perform the rotation for you. Your driver can and should assume that the screen width and height have not changed. Simply flush pixels to the display as normal. Software rotation requires no additional logic in your flush_cb callback.

There is a noticeable amount of overhead to performing rotation in software, which is why hardware rotation is also available. In this mode, LVGL draws into the buffer as though your screen now has the width and height inverted. You are responsible for rotating the provided pixels yourself.

The default rotation of your display when it is initialized can be set using the rotated flag. The available options are LV_DISP_ROT_NONE, LV_DISP_ROT_90, LV_DISP_ROT_180, or LV_DISP_ROT_270. The rotation values are relative to how you would rotate the physical display in the clockwise direction. Thus, LV_DISP_ROT_90 means you rotate the hardware 90 degrees clockwise, and the display rotates 90 degrees counterclockwise to compensate.

(Note for users upgrading from 7.10.0 and older: these new rotation enum values match up with the old 0/1 system for rotating 90 degrees, so legacy code should continue to work as expected. Software rotation is also disabled by default for compatibility.)

Display rotation can also be changed at runtime using the lv_disp_set_rotation(disp, rot) API.

Support for software rotation is a new feature, so there may be some glitches/bugs depending on your configuration. If you encounter a problem please open an issue on GitHub.

API

.. doxygenfile:: lv_hal_disp.h
  :project: lvgl